1. Field of the Invention
The present invention relates generally to vacuum boosters for vehicles. More particularly, the present invention relates to a vacuum booster of brake device, which is adapted for a brake device including an liquid pressure generating unit pumping and supplying brake liquid stored in a reservoir to a wheel cylinder by using a pump through a port that is open to a cylinder of a master cylinder.
2. Discussion of the Related Art
As well known to those skilled in the art, a typical vacuum booster includes a partition member, which is installed in a booster shell so as to be movable forwards and backwards to partition a pressure chamber into a variable pressure chamber and a negative pressure chamber, a valve piston coupled to the partition member, an output member provided in the valve piston so as to be movable forwards and backwards so that a piston of a master cylinder is pushed and moved by forward movement of the valve piston, a reaction applying means for transmitting part of reaction force that is transmitted from the piston to the output member to an input member, and a valve mechanism installed in the valve piston to switch an liquid passage.
The valve mechanism includes a ring-shaped negative pressure valve seat provided in the valve piston, a plunger slidably fitted in the valve piston, a ring-shaped atmospheric valve seat provided in the plunger, a valve body closely contacted with the negative pressure valve seat and the atmospheric valve seat, a negative pressure passage communicating with a space defined outside a negative pressure valve where the negative pressure valve seat contacts with the valve body, an atmosphere passage communicating with a space defined inside an atmospheric valve where the atmospheric valve seat contacts with the valve body, and a variable pressure passage communicating with a space defined between the negative pressure valve and the atmospheric valve.
When the vacuum booster having the above-mentioned construction is not in operation, the valve body comes into contact with the atmospheric valve seat of the plunger and thus closes the atmospheric valve, thereby isolating the atmosphere passage from the variable pressure passage, and is separated from the negative valve seat to open the negative pressure valve, thus causing the negative pressure passage to communicate with the variable pressure passage. In this state, because the negative pressure chamber and the variable pressure chamber, which are formed on opposite sides of the partition member in the vacuum booster, communicate with each other through the negative pressure passage and the variable pressure passage, no pressure difference is applied to the partition member, thus the valve piston and the plunger are maintained at non-operating positions.
When the vacuum booster is in operation, the valve body is moved away from the atmospheric valve seat of the plunger to open the atmospheric valve, thus causing the atmosphere passage to communicate with the variable pressure passage, and is brought into contact with the negative pressure valve seat to close the negative pressure valve, thus isolating the negative pressure passage from the variable pressure passage. In this state, because the variable pressure chamber, which is defined behind the partition member in the vacuum booster, communicates with the atmosphere through the variable pressure passage and the atmosphere passage, a pressure difference is applied to the partition member, therefore the partition member is moved forwards.
Here, in the conventional vacuum booster, the valve mechanism operates such that the input from a brake pedal is balanced with part of the reaction force from the piston of the master cylinder by the operation of the reaction applying means, thus boosting the input. However, the conventional vacuum booster is problematic in that, at the initial stage of the braking operation, as the reaction force is small, and the servo balance of the valve mechanism, in which the negative pressure valve and the atmospheric valve are closed together, is unstable, vibration or noise is generated. In other words, when the reaction force from the piston is small, the force applied in the direction in which the atmospheric valve is closed is small, and therefore the atmospheric valve is not reliably closed, so that outside air is drawn too much into the variable pressure chamber. Thus, subsequently, during a process of balancing the valve mechanism by using the reaction force from the piston and the input from the brake pedal, the negative pressure valve is in open state though the atmospheric valve is closed because the reaction force is increased relative to the input. It results in generating vibration.
Recently, for example, to increase the driving stability of a vehicle, a brake device including an liquid pressure generating unit has been used, which is constructed such that, when the master cylinder is in a non-operating state, brake liquid is pumped and supplied by a pump to a wheel cylinder through a port that communicates with a reservoir and is opened to the cylinder. The brake device usually has a structure such that, when the master cylinder is in a non-operating state, the area, with which the port communicates with the cylinder, is extended so that brake liquid is more rapidly supplied from the reservoir to the wheel cylinder. However, in such a construction, because liquid pressure is not applied to the wheel cylinder until the port is isolated from the master cylinder by the piston that is pushed and moved forwards by the output member of the vacuum booster when the master cylinder is operated, reaction force from the piston is reduced at the initial stage of the braking operation, with the result that the valve mechanism easily vibrates.
In an effort to overcome the above problem, in a vacuum booster disclosed in Japanese Laid-Open Patent Publication No. 2003-127851 (for example, in pages 2 and 3 and FIGS. 1 and 2 thereof), an atmospheric valve, which allows a variable pressure chamber to communicate with the atmosphere or isolates it therefrom, comprises a first atmospheric valve and a second atmospheric valve, and an orifice passage communicating with the atmosphere defined between the first atmospheric valve and the second atmospheric valve. In this construction, when the vacuum booster is in a non-operating state, the first atmospheric valve and the second atmospheric valve are both closed, so that the variable pressure chamber is isolated from the atmosphere. When the operation of the vacuum booster begins, the first atmospheric valve is opened while the second atmospheric valve remains closed, and thus outside air is drawn into the variable pressure chamber through the orifice passage. Thereafter, the second atmospheric valve is opened, so that outside air is drawn into the variable pressure chamber through both the orifice passage and the second atmospheric valve.
Here, because the flow of outside air is limited by the orifice passage, the amount of outside air drawn into the variable pressure chamber is reduced compared to that of the vacuum booster of the related art. Therefore, at the initial stage of the braking operation, the amount of output relative to the amount of input becomes appropriate, thus it can prevent vibration and noise in the valve mechanism. Furthermore, when a second seat portion of a valve body constituting the second atmospheric valve is separated from the first atmospheric valve of a plunger, outside air can be rapidly drawn into the variable pressure chamber, both through the second atmospheric valve and through the first atmospheric valve, which is opened sooner than the second atmospheric valve. Therefore, a braking operation is equal to that of the vacuum booster of the related art can be conducted.
In the vacuum booster of Japanese Laid-Open Patent Publication No. 2003-127851, air passing through the atmospheric valve at the initial stage of the braking operation is restricted by the orifice passage such that outside air is restricted to be drawn into the variable pressure chamber. However, because the flow rate of air is restricted by contraction, it is necessary to greatly increase contraction resistance to prevent vibration and noise in the valve mechanism. As a result, at the initial stage of the braking operation of the vacuum booster, operational responsiveness is deteriorated.